San Francisco — As Facebook expands, with 250 million users posting 1 billion pieces of content every week, the site continues to draw sharp criticism from privacy advocates, lawyers, and governments over how it uses the data that members regularly – and often cavalierly – post onto the site.

This week five California Facebook users joined the chorus of critics. In a lawsuit filed Monday, they charge that Facebook – the Web's dominant social networking ecosystem – unlawfully used their private information or intellectual property without consent.

What's more, they claim, Facebook is merely a data mining and marketing machine that masquerades as a social networking service.

The breakneck growth of the site over the past five years has come with intense scrutiny. Facebook has been criticized for how it has shared user information not only with third-party vendors but also between users not in one another's network. Facebook does collect user information to share with third parties, according to its terms of service.

Some users have complained that their images have been usurped unknowingly and used in advertisements that appear on the site. One married user's image was recently placed on an ad for "hot singles" – an ad that subsequently appeared on her husband's Facebook page.

Only about 20 percent of members use Facebook's privacy settings to control who can view their personal information. But even with strict privacy controls, it's tough to prevent how your images will be used, experts say.

Privacy groups and some of the site's own members have vigilantly monitored Facebook policies. When Facebook changed its terms of service earlier this year – a revision that appeared to give it broad ownership over users' content even after they deleted their accounts – the Electronic Privacy Information Center in Washington threatened to file a complaint with the Federal Trade Commission. Facebook retreated and agreed to allow for greater user participation in drafting new policies.

A Canadian commission has also challenged Facebook over its policies. It recommended that the site make it easier for users to remove accounts and all related personal information. It also asked the company to more explicitly explain to users how their information is being used. Facebook has been working with Canadian officials to meet their recommendations.

But while Facebook has been challenged over privacy issues before, the civil lawsuit filed this week in a California court may be the most blunt attack on the website yet.

The plaintiffs include two minors who lawyers say are too young to understand the site's policies, a photographer who claims her photos have been used on the site without prior consent, a user who complains that the site's policies have changed without her knowledge, and an actress who says her images were distributed on Facebook without consent.

Facebook says its sees "no merit to this suit and we plan to fight it."

Some tech blogs have shrugged off the suit. Helen Popkin, a technology blogger at MSNBC.com, says users shouldn't be too surprised when Facebook posts go public. "That's right, you are responsible for where you choose to put your personal information."

"The plaintiffs are complaining about many of the very mechanisms that make Facebook a social network," Techcrunch says.

But younger Facebook users may not be savvy enough to know what information is suitable for friends and what's appropriate for strangers, writes Ms. Harris of the Center for Democracy and Technology.

While she says that parents should quickly familiarize themselves with Facebook's privacy settings, "the responsibility to protect privacy does not rest with social network users alone. Industry must adopt strong privacy policies, and the government must vigorously police those privacy promises."